Photosensitive elements which can be used to prepare flexographic printing plates generally comprise (1) an addition polymerizable nongaseous ethylenically unsaturated monomer, (2) a photoinitiating system activated by actinic radiation and (3) an elastomeric or thermoplastic-elastomeric polymeric binder comprising polymerized conjugated diene monomers. Photosensitive compositions such as these frequently utilize plasticizers to increase the softness and flexibility of the developed flexographic plates.
Processes for producing a flexographic printing plate from a photosensitive element are well known in the art. Typically, a plate is formed by first, exposing imagewise a photopolymerizable element to actinic radiation. Unexposed areas are subsequently washed off using a suitable developer solvent. The plate is then dried and, if necessary, treated to remove surface tackiness. To ensure final plate hardening and photopolymerization, the printing plate can be further post exposed. Following development, the printing plate is ready to be mounted on a cylinder and the printing process is initiated.
It has been found that during printing, the thickness of the plates may decrease. It is believed that the decrease in plate thickness may be due to prolonged exposure to the solvents in the printing inks which leach out materials in the plate such as plasticizers and unreacted monomer. This thickness loss results in poorer print quality as the printing run progresses. It is particularly deleterious to critical four-color printing. It is not possible to correct this by simply removing the plasticizer because plates without plasticizer are too hard and have insufficient print quality. Nor can this problem be solved by reducing the exposure time to the solvents in the printing inks because this would impact on print quality as well as reduce the number of copies during printing.